Problem 76 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following compounds on reaction with \(\mathrm{NaOH}\) and \(\mathrm{Na}_{2} \mathrm{O}_{2}\) gives yellow colour? (a) \(\mathrm{Zn}(\mathrm{OH})_{2}\) (b) \(\mathrm{Cr}(\mathrm{OH})_{3}\) (c) \(\mathrm{Al}(\mathrm{OH})_{3}\) (d) none of these

Step-by-Step Solution

Answer

Option (b) \( \mathrm{Cr}(\mathrm{OH})_{3} \) gives yellow color with \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \).

Step 1: Identify Reactants and Products

First, we identify the compounds given: (a) \( \mathrm{Zn}(\mathrm{OH})_{2} \), (b) \( \mathrm{Cr}(\mathrm{OH})_{3} \), (c) \( \mathrm{Al}(\mathrm{OH})_{3} \), and determine how they react with \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \). Specifically, \( \mathrm{Cr}(\mathrm{OH})_{3} \) can react with \( \mathrm{NaOH} \) forming a yellow colored chromate compound.

Step 2: Evaluate Reaction with NaOH and Na2O2

Focus on \( \mathrm{Cr}(\mathrm{OH})_{3} \). When \( \mathrm{Cr}(\mathrm{OH})_{3} \) is treated with excess \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \), the chromium(III) hydroxide is oxidized to chromate (\( \mathrm{CrO}_{4}^{2-} \)), which imparts a yellow color to the solution.

Step 3: Analyze Results for Yellow Color

The key to solving the problem is recognizing the compound that forms a noticeable yellow chromate ion. Since reaction with \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \) specifically oxidizes \( \mathrm{Cr}(\mathrm{OH})_{3} \) into chromate (yellow in color), option (b) is correct.

Key Concepts

Chemical Reactions

When substances interact to form new products, they undergo a fascinating process known as a chemical reaction. A chemical reaction involves breaking original bonds and forming new ones, resulting in new substances with different properties.
In the context of the exercise, we're dealing with the hydroxides of metals reacting with sodium hydroxide (\( \mathrm{NaOH} \)) and sodium peroxide (\( \mathrm{Na}_{2} \mathrm{O}_{2} \)). These interactions are specific as the accompanying conditions like temperature and the presence of other compounds can dictate the chemistry occurring.
Particularly when \( \mathrm{Cr}( ext{OH})_3 \) interacts with \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \), a transformation occurs leading to the formation of a chromate ion. This change not only alters the chemical composition but also imparts a distinctive yellow color to the solution. Such color changes are indicators of chemical transformations, revealing the presence of specific ions or compounds.

Oxidation

Oxidation is a chemical phenomenon where an element undergoes a change due to a shift in its electron configuration. Specifically, oxidation involves the loss of electrons. This process is crucial in chemical reactions, including those in biological systems, industrial processes, and environmental transformations.
In our exercise, oxidation plays a central role. The \( \mathrm{Cr}( ext{OH})_3 \) is oxidized when it reacts with \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \). This means the chromium present in the compound moves to a higher oxidation state, transforming into a chromate ion: \( \mathrm{CrO}_{4}^{2-} \).
Such reactions are integral in fields like metallurgy, where changing the oxidation state of metals affects their properties like color, solubility, and reactivity. In analytical chemistry, observing these changes can indicate the presence of certain reagents or help in qualitative analysis.

Chromate Formation

Chromate ions are yellow-colored polyatomic ions formed from chromium in a hexavalent state. They are often denoted as \( \mathrm{CrO}_{4}^{2-} \). The formation of chromates is significant in various industrial applications including pigments and dyes.
In the exercise, chromate formation occurs when \( \mathrm{Cr}( ext{OH})_3 \) reacts with \( \mathrm{NaOH} \) and \( \mathrm{Na}_{2} \mathrm{O}_{2} \), resulting in yellow-colored solutions. This is because the oxidation of chromium from a trivalent to a hexavalent state transforms \( \mathrm{Cr(OH)}_3 \) into \( \mathrm{CrO}_{4}^{2-} \).
Chromate compounds are known for their vivid colors, making them useful in dyeing and as indicators of chemical reactions. However, care must be taken with chromates as they are also considered hazardous. Nonetheless, understanding chromate chemistry allows for their safe and effective use in numerous applications.